First off, I think this needs to be said. I am very glad you are among the living. I lost a lover once to suicide, and it nearly drove me mad.
Epona is indeed the first horse-deity that comes to mind--she is the source of dreams and nightmares. Night-mare, referencing the horse, the dark mare that comes in the night and sends you terrible dreams.
Notwithstanding the strange and malevolent associations that correspond with Epona, I can't help but think you've received an entirely different encounter. A positive encounter.
You saw a white horse, not a dark one. A savior, a benefactor. The white horse is the messenger, the protector, the defender and supporter.
The symbol retains power even in recent literature. J. R. R. Tolkien's Shadowfax, a white horse, the king of horses, is instrumental to the ultimate climax of destroying a great evil. Peter S. Beagle's The Last Unicorn was, in essence, about a white mare saving the world fro being devoured by greed and evil. Going back further, Pegasus, the winged horse of Greek mythology, was pure white. Odin's eight-legged steed Sleipnir, the fastest and best of horses, was also said to be pure white.
As for other horse-gods other than Epona: Rhiannon (Celtic goddess) is said to ride a pale white horse. So did the Svantovit, the Slavic god of war and fertility.
If a white horse saved you, as you say, that is deeply symbolic. White horses bear gods, priests, and heroes. They are ghosts and nightmares, yet they are powerful and benign. Swift, elegant, capable of great endurance. I think the fact that you were saved by a white horse means that you need to live. You are destined for something.
Please, if you can, find someone to talk to if you still have suicidal ideation. The horse is also a symbol of communication--perhaps talking to someone will help?
In any case, good luck to you. I hope my rambling post helps.
